The present invention generally relates to fabrication of optical devices and more particularly to a fabrication process of an optical device supplied with an output optical beam from a laser diode for generating a second harmonic optical beam therefrom.
In the wide-spread use of laser diodes in various fields such as laser printers, laser scanners, optical disk devices, etc., there is an increasing demand for an optical beam with a reduced wavelength. Thus, there are intensive efforts for reducing the oscillation wavelength of laser diodes from the conventional infrared region to the visible region for increased storage capacity as well as for easiness in handling the device. On the other hand, there is a limitation in the extent of reduction of oscillation wavelength in the currently available techniques, and attempts are made to use the higher order harmonics of the laser oscillation for the formation of desired short-wavelength output beam.
Conventionally, the bulk crystal of materials that exhibit a non-liner optical property has been used for converting the optical beam of the fundamental mode to the second harmonic beam. Such a device is known as an SHG (second-order harmonic generation) device. Unfortunately, the optical non-linearity that these bulk crystals show is generally limited and the efficiency of conversion is small. Associated therewith, there exists a problem that one needs a large optical output power for the laser diode that drives the SHG device, in order to obtain the desired power for the second-harmonic optical beam.
Meanwhile, there is a proposal to use a ferroelectric material for achieving the desired conversion of wavelength of the optical beam with an improved efficiency of conversion. Generally, the ferroelectric materials exhibit very large non-linear optical constants.
FIG. 1(A) shows a conventional SHG device 111.sub.A of so-called Cherenkov radiation type that uses a substrate 112 of a ferroelectric crystal such as LiNbO.sub.3, LiTaO.sub.3, etc., and an optical waveguide 113 is provided on the surface of the substrate 112. The optical waveguide 113 is formed for example by a modification of the composition of the substrate and has an increased refractive index for confining the optical beam therein. Such a waveguide 113 may be formed by processing the substrate surface by benzoic acid or pyrophosphoric acid for substituting protons for the cations of the ferroelectric material. An input optical beam is injected at an end of the optical waveguide 113 such that the optical beam travels along the optical waveguide 113, and the second harmonic optical beam is formed outside the optical waveguide. In such a device, there exists a problem in that the second order beam has an arc-shape cross section as indicated in the drawing and the optical beam having such an arc-shape cross section is difficult to focus into a small beam spot. Thus, the device of this prior art is not suitable for the applications where a small, sharp optical beam is required as in the case of information processing apparatuses.
FIG. 1(B) shows another conventional device 111.sub.B called the QPM (quasi-phase matching) type that also uses a ferroelectric substrate 112 formed with an optical waveguide 113 similarly to the previous device 111.sub.A, wherein the substrate 112 is polarized in a predetermined direction. In the device 111.sub.B, it should be noted that there are formed a number of regions 115 with a polarization opposite to the polarization of the substrate 112 along the direction of the optical waveguide 113, and such an inversion region 115 is repeated with a predetermined pitch such that a non-inversion region 114 having the polarization identical with the substrate is interposed between the adjacent regions 115. There, the pitch of the regions 114 and hence the pitch of the regions 115 are set such that the decay of the second-harmonic optical beam due to the refractive index difference between the fundamental mode and the second-harmonic mode in the optical waveguide 113 is compensated for particularly at the wavelength corresponding to the second harmonic mode. By injecting the input optical beam having the fundamental wavelength at an end of the optical waveguide 113, the intensity of the second-harmonic beam increases as the optical beam travels along the waveguide 113. The device 111.sub.B has a preferable feature, associated with the fact that the desired second-harmonic optical beam is confined in the optical waveguide 113, in that the distortion of the beam shade is minimum. Thereby, one can obtain a very sharp optical beam by focusing the second-harmonic beam by suitable optical means.
Various efforts are made so far for developing the process for inducing the inversion of polarization in the ferroelectric substrate such that the inversion regions 115 are formed with a controlled pitch. For example, the region 115 may be formed by: (a) diffusing titanium into the LiNbO.sub.3 substrate selectively in correspondence to the regions where the desired inversion of polarization is to be induced; (b) removing the Li component from the surface of the LiNbO.sub.3 substrate selectively in correspondence to the regions where the inversion of polarization is to be induced: (c) exchanging the Li.sup.+ ions in the LiTaO.sub.3 substrate with protons in correspondence to the regions where the desired inversion of polarization is to be induced and heating the substrate thus processed; and (d) irradiating the substrate by an electron beam in correspondence to the regions where the desired inversion of polarization is to be induced.
FIGS. 2(A) and 2(B) show the conventional process for causing the inversion of polarization in accordance with the foregoing process (a), wherein a titanium layer 121 is deposited on the surface of a substrate 112a of LiNbO.sub.3 by a vacuum deposition process, and the titanium layer 121 thus deposited is patterned as shown in FIG. 2(A) such that the titanium layer remains only in correspondence to the parts on which the regions 115 are to be formed. Next, the structure of FIG. 2(A) is subjected to a heat treatment process at a temperature immediately below the Curie temperature of the substrate (1210.degree. C.) for causing a diffusion of titanium into the substrate as shown in FIG. 2(B). As a result of the diffusion, each inversion regions 115 are formed with a triangular cross section.
FIGS. 3(A) and 3(B) show another conventional process for forming the polarization inversion according to the process (b), wherein a silicon oxide film 122 is provided on the surface of the LiNbO.sub.3 substrate 112a by sputtering and patterned subsequently to form the structure of FIG. 3(A). Further, the structure of FIG. 3(A) is held at a temperature immediately below the Curie temperature of the substrate for causing a diffusion of Li into the silicon oxide film 122. Thereby, the composition of the substrate 112a is modified selectively in correspondence to the parts where the diffusion has occurred, and the desired inversion of polarization occurs in correspondence to the regions thus formed with the inversion of polarization.
In any of these conventional processes, there arises a problem, associated with the relatively low threshold of optical damage of the LiNbO.sub.3 substrate, in that the substrate as well as the optical waveguide may be damaged by the optical radiation. Further, the depth of the regions 115 formed by the foregoing processes is too small for causing the desired interaction with the input optical beam propagating through the optical waveguide.
With respect to the problem of the optical damage, the LiTaO.sub.3 substrate is advantageous because the large threshold of optical damage pertinent to LiTaO.sub.3. In addition, the LiTaO.sub.3 substrate is advantageous from the view point of forming the regions 115 with a relatively large depth that can reach as much as one quarter of the pitch formed by the regions 115
On the other hand, the optical waveguide 113 that is formed in the LiTaO.sub.3 substrate generally has a thickness or depth, measured perpendicularly to the upper major surface of the substrate, of more than 2 .mu.m. The formation of the optical waveguide 113 having a depth smaller than 2 .mu.m is difficult or not desirable because of various reasons. In order to achieve the conversion of the fundamental optical beam to the second-harmonic beam by the interaction with the regions 115 effectively, therefore, it is necessary that each inversion regions 115 extends vertically from the surface to a sufficient depth, larger than about 2 .mu.m and preferably larger than 10 .mu.m.
FIGS. 4(A)-4(C) show the conventional process for forming the polarization inversion regions 115 on the surface of a LiTaO.sub.3 substrate 112c according to the foregoing process (c), wherein the surface of the substrate designated as +C represents the lower major surface to which the polarization vector is pointed, while the surface designated as -C represents the upper major surface opposing the +C surface.
Referring to FIG. 4(A), a Z-cut crystal of LiTaO.sub.3 is used as the substrate 112c with a polarization formed such that the polarization vector points toward the lower major surface +C of the substrate 112c. In other words, the lower major surface +C of the substrate 112c provides the +C surface while the upper major surface of the substrate provides the -C surface in the illustrated example. Further, the upper major surface, i.e. the -C surface, is polished to the optical grade and a mask layer 123 of Ta is deposited on the -C surface. The mask layer 123 is patterned such that there are formed openings 123a, in correspondence to the inversion regions 115 to be formed to expose the upper major surface of the substrate 112c. As illustrated in FIG. 4(A), each opening 123a has an elongated elongate form extending in the Y-direction or lateral direction of the substrate 112c. Further, the opening 123a is repeated a plurality of times in the X-direction in correspondence to the periodical repetition of the inversion regions 115.
Next, in the step of FIG. 4(B), the substrate of FIG. 4(A) thus masked with the Ta mask 123 is subjected to a proton exchange process. More specifically, the structure of FIG. 4(A) is immersed in a solution of pyrophosphoric acid held at 250.degree. C. for 30 minutes. As a result of the proton exchange process, the Li.sup.+ ions in the substrate are exchanged with protons H.sup.+ via the openings 123a.
After the step of FIG. 4(B), the structure of FIG. 4(B) is subjected to a heat treatment process at a temperature immediately below the Curie temperature of the substrate 112c (about 590.degree. C.) for causing an inversion of polarization in correspondence to the regions where the proton exchange has occurred, and the inversion regions 115 are formed periodically but with a separation from each other by the non-inversion regions 114 as shown in FIG. 4(C). After the inversion regions 115 are formed, the substrate 112c is formed with an optical waveguide 113 (not shown in FIGS. 4(A)-4(C)) also by the proton exchange process such that the optical waveguide 113 extends in the X-direction, or longitudinal direction, of the substrate 112c.
In the process (c) explained above, one can form the inversion regions 115 with a width substantially identical with the width of the non-inversion regions 114 and further a with a semi-circular cross section when viewed in the Y-direction of the substrate 112c.
When such a structure is applied to the conversion of the laser beam having a wavelength of about 1 .mu.m, it is necessary to provide the inversion regions 115 with a pitch of about 6 .mu.m. This means that each inversion regions 115 should have a width of about 3 .mu.m. When the width is about 3 .mu.m, the depth of the regions 115 formed in accordance with the process of FIGS. 4(A)-4(C) has a depth of about 1.5 .mu.m in correspondence to one-quarter of the pitch. In view of the depth of 2 .mu.m of the optical waveguide 113, the inversion regions 115 do not reach the bottom of the optical wave guide 113, and the SHG device fabricated according to the process of FIGS. 4(A)-4(C) cannot meet the demand of efficient conversion of the fundamental optical beam. This problem of insufficient depth of the regions 115 becomes particularly acute when the SHG device is used in combination with a laser diode that produces an output optical beam with a reduced wavelength such as 0.85 .mu. m. In such a case, the desired pitch of the inversion regions 115 for the wavelength conversion is about 3-4 .mu.m. This means that one can form the inversion regions 115 only with a depth of less than 1 .mu.m. Thereby, the SHG device is no longer effective in conversion of the fundamental optical beam to the second order optical beam.
In the fourth process (d) for forming the inversion regions 115 by the electron beam, one can achieve a sufficient depth for the regions 115. On the other hand, such a process for writing the pattern in correspondence to the inversion regions 115 by the focused electron beam requires a long processing time and is disadvantageous from the view point of mass producing the SHG device.